Fructose is an isomer of the hexose (C6) aldose sugars glucose and mannose. Glucose is the most abundant monosaccharide in nature and the cheapest hexose. Cellulose as the structural component of the primary cell wall of green plants and many forms of algae consists of a linear chain of several hundred to over ten thousand β(1→4) linked D-glucose units and therefore a unlimited source of glucose for isomerization to fructose. Fructose is widely used in the food industry as sweetener (high-fructose corn syrup, HFCS), since it contributes many useful physical and functional attributes to food and beverage applications. On the other hand, one attractive approach to convert biomass into biofuels and feedstock chemicals is the direct conversion of hexose carbohydrates into 5-hydroxymethylfurfural (HMF) and/or levulinic acid in aqueous media or levulinate esters in the presence of an alcohol. In contrast to glucose, fructose readily dehydrates to form HMF using an acid catalyst followed by the etherification with an alcohol.
Traditionally, the equilibrium limited isomerization of glucose to fructose has been carried out industrially in the presence of the enzyme glucose/xylose isomerase. To achieve high enzyme specificity without formation of side products the reaction requires ambient pH conditions and temperature. However, from an economic point of view, the activity of enzymes is still low and large quantity of enzyme is thus needed. Moreover, irreversible deactivation of the enzyme may occur. Recently, a combined use of ultrasound irradiation and ionic liquids has been studied by Wang et al. (Wang, Y.; Pan, Y.; Zhang, Z.; Sun, R.; Fang, X.; Yu, D. Process Biochemistry 2012, 47, 976) to improve the reaction rate and product yield in enzymatic isomerization of glucose to fructose allowing achieving a fructose yield of 45.3%.
Alternatively to enzymes, glucose can be transformed into fructose by aldose-ketose isomerization, in the presence of a base. However, the monosaccharides are unstable in alkaline media and a high amount of by-products are produced due to side reactions. Generally, Bronsted acids are not efficient catalysts for aldose isomerization, although the efficacy may be a function of reaction conditions (Kruger, J. S.; Nikolakis, V.; Vlachos, D. G. Current Opinion in Chemical Engineering).
Xylulose is an isomer of the pentose (C5) sugar xylose. Xylose is a precursor of hemicellulose, which comprises about 30% of plant matter. Wood and other plant materials provide unlimited sources of xylose and its precursors. Another 45% of the plant material is cellulose, 15% are lignin and about 10% are ash. Xylulose may be used as a chemical platform for different enzymatic or chemical processes or converted, e.g. by fermentation to ethanol and used as biofuel. Apart from this, xylulose is an intermediate to form furfural from xylose. Furfural is one of the important platform chemicals, which can be used to produce a verity of chemicals such as furfural alcohol, 2-methyl furan, furan, tetrahydrofuran, furfuryl amine, etc.
Among solid acid catalysts, zeolites have widely been used in the petroleum industry because of the many advantages they present. As heterogeneous catalysts, zeolites do not require costly post-reaction separation processes that are needed for many homogeneous catalysts, and they can be used under a wider range of reaction conditions than biocatalysts.
Zeolites are tridimensional crystalline aluminosilicates with the following formula in the as-synthesized form: xM2/nO.xAl2O3.ySiO2.WH2O where M is a cation which can belong to the group IA or HA or can be an organic cation, while n is the cation valence, and W represents water contained in the zeolite voids. Crystalline structures of the zeolite type but containing tetrahedrally coordinated Si, Al, P, as well as transition metals and many group elements with the valence ranging from I to V such as, Sn, B, Ga, Fe, Cr, Ti, V, Mn, Co, Zn, Cu, Sr, etc., have been synthesized with the generic name of zeotypes, including AlPO4, SAPO, MeAPO, and MeAPSO type molecular sieves. The main characteristic of the zeolites and zeotypes is that the tetrahedral primary building blocks are linked through oxygen producing a three-dimensional network containing channels and cavities of molecular dimensions.
The channel sizes are conventionally defined as ultralarge pore materials (>12-membered rings) with a free diameter above 8 Å, large (12-membered rings) with a free diameter of about 6-8 Å, medium (10-membered rings) with a free diameter of about 4.5-6 Å, or small (8-membered rings) with a diameter of about 3-4.5 Å, depending on the smallest number of O, Al and Si atoms that limits the pore aperture of their largest channel. Examples of zeolites and zeotypes with different pore size may be found in Chemical Reviews, 95 (1995) 559-614. The system of channels in these molecular sieves produces solids with very high surface area and pore volume, which are capable of adsorbing great amounts of substrate/reactants. This fact combined with the possibility to generate active sites inside of the channels and cavities of zeolites and zeotypes produces a very unique type of catalyst, which by itself can be considered as a catalytic microreactor.
Zeolites containing tetravalent metal atoms in tetrahedral coordination modes have been explored as solid Lewis acid catalysts. Recent studies have shown that metal centers are highly active for the isomerization of glucose. In particular, the Lewis acid zeolite Sn-BEA has been shown effective for catalyzing the isomerization of a series of C5 and C6 sugars: dihydroxyacetone (DHA), glyceraldehyde, and glucose with activities comparable to biological processes by a mechanism similar to enzymatic catalysts (5-8). However, Sn-BEA is cumbersome to synthesize and incorporate tin metal, which is a toxic heavy metal and therefore potentially dangerous to the environment and down-stream products.